Description of the Related Art
Image forming apparatuses, such as a copying apparatus, a printer, and a facsimile apparatus, include a process for forming an image on a heating target, such as a sheet of regular paper and OHP paper. In such image forming apparatuses, although various image formation methods are employed, an electro-photographic method is widely adopted from viewpoints of speed, image quality, cost, and so on.
In the electro-photographic method, a toner image that is to be fixed is formed on the heating target, such as a sheet of regular paper and OHP paper, and a fixing process fixes the toner image on the heating target by heat and pressure applied by a fixing apparatus. As the fixing apparatus, a heat roller is widely adopted for rapidity and safety.
The fixing apparatus adopting the heat roller includes a nip part that is constituted by a heating roller heated by a heating unit, such as a halogen heater, and a pressurization roller that counters the heating roller. The heating target is passed between the heating roller and the pressurization roller such that the toner image on the heating target is fixed by heat and pressure. The nip part carries out pressure welding.
When fluoride system resin as a release agent layer covers the metal core of the heating roller of the fixing apparatus of the heat roller method, since the fluoride system resin is hard, a problem of image quality arises as follows. The toner image on the heating target has microscopic unevenness. If the surface of the heating roller is hard, the surface cannot follow the unevenness, and microscopic compliance with the uneven surface of the heating roller becomes low. For this reason, the toner image after being fixed to the heating target contains uneven gloss between a portion where the heating roller makes contact, and a portion where the heating roller does not make contact.
In conventional monochrome copying apparatuses, since required quality of an image is not so high as compared with full color copying apparatuses, the heating roller including the core metal covered with fluoride system resin is acceptable. However, as the speed of the apparatuses is raised, and a monochrome copying apparatus is used for printing, requirements for high-definition production are becoming high.
On the other hand, requirements for producing high-definition images are higher for full color copying apparatuses than for the monochrome copying apparatuses. A high quality fixed image without uneven gloss is obtained by providing close contact between the surface of the heating roller and a toner layer on the heating target, which is realized by covering the core metal of the heating roller with an elastic layer (heat-resistant rubber), the elasticity of the rubber of a heating roller providing the close contact. This technology has been applied to monochrome copying apparatuses.
However, the metal core of the heating roller is made of a metal, such as iron and aluminum, having a high heat capacity. For this reason, the heat roller method has a shortcoming in that it takes a long starting time of several minutes, sometimes longer than ten minutes, for the temperature of the heating roller to rise to about 180 degrees C.
To cope with this problem of the image forming apparatus, power is continuously supplied to the heating roller, even if a user does not use the image forming apparatus, i.e., during standby, such that the temperature of the heating roller is maintained at a preheating temperature, which is set at a little lower than the operational temperature, so that the temperature can be quickly raised to the operational temperature when the heating roller is used. While this solution shortens the waiting time of the user, excessive energy is wasted during the standby period. In addition, an investigation report says that the consumption of energy during the standby period often ranges about 70 to 80 percent of the consumption energy of the image forming apparatus in operation.
Recently and continuing, energy-saving regulations are enacted from the rise of environmental protection consciousness in countries worldwide. In Japan, the Law concerning the Rational Use of Energy is being revised and strengthened, and in the U.S., energy-saving programs, such as energy star and ZESM (Zero Energy Star Mode), are being enacted. In order to meet these regulations and programs, it is desirable to suspend the power supply to the heating roller while the image forming apparatus is in the standby mode. Given that the power consumption during the standby mode is considerably high, such suspension will greatly contribute to power-saving.
However, if the power is not supplied to the heating roller during the standby mode in the case of the conventional fixing apparatus, it takes the long time for the temperature of the heating roller to rise at the time of reuse, and the long waiting time reduces user-friendliness. For this reason, an energy-saving type image forming apparatus wherein the temperature of the heating roller quickly rises is desired. For example, ZESM requires a re-starting time of 10 seconds or less.
In order to shorten the temperature rising (heating) time of the heating roller, it is effective to lower the heat capacity of the whole fixing apparatus including the pressurization roller. Japanese Provisional Patent No. H11-133776 discloses a fixing apparatus that realizes high-definition image production, improvement in speed, energy saving, and long service life by preparing a fixing roller containing an elastic layer, a pressurization belt constituting a nip part, and a pressurization unit arranged inside the pressurization belt, wherein a heating target is passed between the fixing roller and the pressurization belt.
Further, Japanese Patent No. 2001-92281 discloses a fixing apparatus that fixes a toner image on a transfer medium by heating and pressurization, providing high definition, energy saving, and a long service life, which includes:
a film-like rotational unit that is prepared enclosing a fixed heating element, and
a rotational unit having a roll-like structure for heat ray fixing, which further includes a heat ray irradiation unit for emitting heat rays installed countering the film-like rotational unit, a transparent cylindrical unit that transmits the heat rays, a transparent elastic layer prepared outside of the transparent cylindrical unit, and a heat ray absorption layer for absorbing the heat rays prepared outside of the transparent elastic layer.
The temperature rising time of the heating roller can be shortened by increasing injection energy per unit time, i.e., rated power, provided to the heating element that heats the heating roller. In fact, high-speed image forming apparatuses using a power supply voltage of 200 V are available, wherein the temperature rising time of the heating roller is shortened. However, in Japan, generally available commercial power supply is at 100 V 15 A, and a 200 V power supply is available only after a special installation. Thus, expecting a voltage higher than 100 V is not realistic.
Further, image forming apparatuses that raise the total power injected to the heating element of the fixing apparatus, using two systems of the commercial power supply of 100 V 15 A are also available. However, availability of two separate power line systems is not common.
Furthermore, when the supply power to the heating element of the fixing apparatus is simply increased, safety precautions become more important. The temperature of the heating roller rises quickly as a result of supplying high power to the heating element. When a system hangs up, and control of the supply power to the heating element becomes impossible, the probability of ignition becomes considerably high. If the temperature rise of the heating roller is too quick, the temperature of the heating roller may exceed the ignition temperature of paper before safeguards, such as a temperature fuse and a thermostat, operate.
As mentioned above, conventionally, there is a limit to the amount of the injection energy for raising the temperature of the heating roller in a short time.
In order to realize energy savings when increasing the maximum power supplied to the heating element, using an auxiliary power supply for supplying power to the heating element is proposed, wherein a rechargeable battery is used as the auxiliary power supply. As the rechargeable battery, a lead storage battery, a NiCd battery, etc., are typical ones.
However, since it takes several hours to fully charge the rechargeable battery, the problem is that it cannot be used repeatedly in a day. Further, the rechargeable battery is deteriorated through repeated recharging, the capacity being decreased, and has the nature that the greater is the discharge current, the shorter the service life becomes. In the case of a NiCd battery, which is generally considered to have a long service life and being capable of providing a large current, the number of times of recharging is about 500-1000. If recharging is performed 20 times a day, the service life is about a month. Accordingly, time and effort for battery replacement are required, and operating costs, such as battery costs, become high. Further, since it takes a long time to charge the rechargeable battery, recharging is often performed at night, with the rechargeable battery being taken out of the apparatus. Further, the rechargeable battery is capable of discharging little by little, but it has difficulty providing high power for a short duration. Further, if charging is continued without discharging, gas is generated, causing a failure and being unsafe. Furthermore, the lead storage battery uses liquid sulfuric acid, which is not desirable for use in an office apparatus. Due to the shortcomings as described above, it is practically difficult to employ a rechargeable battery for supplying power to the heating element.
In order to solve the shortcomings of the rechargeable battery, proposals have been made that a mass capacitor, such as an electric double layer capacitor, be used by the fixing apparatus, as an auxiliary power supply. In the case of the mass capacitor, the number of times of recharging is almost unlimited, with almost no degradation of charging characteristics, dispensing with periodic maintenance. Further, the mass capacitor can be charged in a short period of time, such as from several seconds to dozens of seconds, which compares favorably with the rechargeable battery requiring several hours of charging time. Further, the electric double layer capacitor is capable of supplying a large current, such as dozens of amperes to hundreds of amperes, which enables power supply in a short time. Further, the mass capacitor does not generate gas and the like, and is safe even when charging is continued. Furthermore, since stored energy of the electric double layer capacitor automatically declines as electric discharge is carried out for a predetermined time, voltage falls, and power supplied is reduced, which provides high safety.
As described above, if a capacitor is used as the auxiliary power supply, power greater than the power that the commercial power supply can provide becomes available to the fixing apparatus during a short time of several seconds to dozens of seconds when the fixing apparatus is heated. Further, since the mass capacitor uses up the stored energy in a short period of time, the power available is reduced after the predetermined time from the start of the electric discharge, realizing a safe configuration of the heating roller, which is not excessively heated. In this manner, a fixing apparatus featuring a short starting time, reliability, durability, and high safety is realized.
Japanese Provisional Patent No. H5-232839 discloses a heating apparatus wherein an auxiliary power supply provides power to a second heating element, rather than increasing the power to a first heater for heating the fixing roller.
Japanese Provisional Patent No. H10-10913 discloses an energy-saving type fixing apparatus that employs an auxiliary power supply. With this fixing apparatus, the rechargeable battery serving as the auxiliary power supply is provided in order to obtain two levels of power from a single power supply. It does not aim at supplying power greater than the power available from only the main power supply.
Japanese Provisional Patent No. H10-282821 discloses an image forming apparatus that uses an auxiliary power supply, such as a rechargeable battery and a primary battery, in addition to the main power supply for providing various functions.
Japanese Provisional Patent No. 2000-315567 discloses a heating apparatus using a mass capacitor in addition to the main power supply as an auxiliary power supply. According to this heating apparatus, the auxiliary power supply assists the commercial power supply at the time of starting; thereby heating time is shortened, saving energy.
Japanese Provisional Patent No. 2000-075737 discloses an image forming apparatus equipped with a power supply based on the commercial power supply and a storage battery, including storage battery checking means for determining presence of the storage battery, and charge capacity surveillance means for supervising charging capacity of the storage battery, wherein productivity is reduced during the charging of the storage battery based on determinations of the storage battery checking means and the charge capacity surveillance means.
Further, according to Japanese Provisional Patent No. 2000-075737, charging a storage battery is carried out externally and during night hours, for charging the storage battery takes a long time.
As a fixing system that realizes the temperature rise of the image forming apparatus in a short period of time, there is a configuration such that a heat-resistant resin film is wound around the circumference of a plate-like ceramic heater. Since the heat capacity of the ceramic heater is made small in this manner, the starting time is shortened. The configuration is put in practical use with low speed image forming apparatuses that deliver 30 sheets a minutes or less.
However, when the configuration is to be applied to a high-speed image forming apparatus, the heat-resistant resin film (the film) has to be thick such that the film is prevented from breaking. Since thermal conductivity of the resin is less than metal, the temperature of the film has to be raised before the film is fed into the nip part, otherwise the heat cannot be transmitted to the heating target in the nip part. For this reason, the area of the plate-like part of the heater becomes large, and high power is required to quickly raise the temperature.
Objective of the Invention
With a fixing apparatus and a heating apparatus using the mass capacitor mentioned above as an auxiliary power supply, the following problems are now clear.
In order to shorten the starting time, while reducing the heat capacity of the fixing roller (heating roller), it is necessary to provide high power to the fixing roller. Then, in order to obtain high power from the auxiliary power supply, a high voltage is more desirable than a large current in view of the load of wiring and a circuit.
However, in a case that an auxiliary power supply employing a mass capacitor is used, and the fixing roller temperature is controlled by turning on/off the power supply, high power is supplied to the heater, which causes sharp changes of the temperature of the fixing roller, as shown by FIG. 4. Accordingly, when the temperature of the fixing roller changes in the middle of fixing an image on the heating target, unevenness of image quality develops, and the image quality is degraded.
As mentioned above, a heating roller, having a core metal covered by an elastic layer (heat-resistant rubber) is available, which prevents gloss unevenness from occurring, and provides a high quality image. However, the elastic layer has poor thermal conductivity, and as many sheets are processed, the surface temperature of the heating roller tends to fall, causing poor fixing. In order to avoid this poor fixing, some image forming apparatuses secure fixing quality by reducing process speed, when the surface temperature of the heating roller becomes lower than a predetermined temperature. Thus, the poor thermal conductivity of the elastic layer of the heating roller works against the speed.
Further, in order to use up the energy that the mass capacitor holds at the starting time that lasts several seconds to dozens of seconds, a configuration that takes out high power from the mass capacitor is required. Since the power=voltage×current, high power can be obtained from the mass capacitor by making output voltage of the mass capacitor high, and increasing the output current of the mass capacitor.
However, the maximum current of a halogen heater that is usually used for heating of the heating roller is about 10 A through 12 A, and it is difficult to increase the maximum current. This is because the life of the halogen heater becomes short if a large current is supplied to the halogen heater. Therefore, in order to supply high power to the halogen heater, the voltage needs to be raised.
However, the mass capacitor has an inherent characteristic in that the voltage per one capacitor cell is as low as about several volts, a little more than 1 V in the case of a hydro-system, and a little less than 3 V in the case of an organic system. The low voltage is for preventing an electrolytic solution from forming inside the capacitor cell of the mass capacitor. For this reason, when the halogen heater conventionally used is to generate heat for heating, dozens of the mass capacitor cells are connected in series to make a power supply unit capable of supplying about 50 V through 100 V to the halogen heater.
Installing the power supply unit of a high voltage in the apparatus, however, poses the following problems. Although an access to the inside of the apparatus is in many cases performed by a maintenance person, a power supply terminal may be inadvertently touched during maintenance work, and an electric shock accident may occur. Further, it is conceivable that a general office worker accesses inside the apparatus for removing a jammed sheet of paper, and the like. For this reason, a preventive measure against an electric shock is required.
Further, as the storage capacity of a capacitor cell of the mass capacitor is becoming large, the number of the capacitor cells to be connected in series for obtaining the high voltage and high power is decreasing, and the fewer number of capacitor cells are capable of raising the temperature of the heating target. However, in order to obtain the high voltage using the mass capacitor, it is necessary to increase the number of the capacitor cells, and in other words, an excess capacity of the capacitor cells has to be provided as the configuration of the power supply unit. At present, since the energy density of the mass capacitor is still low, the size is large, and the cost is still high, it is essential to reduce the number of capacitor cells.
That is, where a halogen heater is employed as the heating element, in order to raise the supply voltage to the halogen heater, capacitor cells capable of providing excess energy are needed, and the power supply for supplying power to the halogen heater becomes large in size and high in cost.
Further, another important objective is related to preventing an overshoot of the temperature. At present, a thermistor is used for detecting the temperature of the fixing roller. Although the size of the thermistor is quite small and reaction speed is improved, the temperature detecting speed of the thermistor is still low for the configuration where power supplied to the halogen heater is high, and the temperature rises quickly. Thus, the overshoot of the temperature is another problem to be solved.